High temperature corrosion is a chemical process wherein high temperature (typically temperatures above 350° C.) causes a material to react with its surroundings, typically with gaseous compounds in the atmosphere, thereby damaging the material. A distinction can be made between uniform surface corrosion, wherein the damage from corrosion is essentially evenly spread over the entire surface of the material, and pitting corrosion, wherein corrosion is more localized and leads to small holes in the material.
High temperature corrosion may be the result of oxidation with molecular oxygen (e.g. scale formation). However, it may also occur in an atmosphere with low oxygen content, for example in a carbon containing atmosphere, for instance an atmosphere that contains carbon monoxide.
High temperature corrosion may for example occur if a metallic object is brought into contact with a gas containing carbon monoxide, methane and/or other carbon-containing components at high temperatures, in particular when said gas has a low oxygen content. Examples of such high temperature corrosion are carburization and metal dusting.
Carburization is a corrosion process wherein a metallic material (typically iron or an alloy thereof, such as steel) is damaged due to exposure to a carbon containing atmosphere at high temperatures. In this process, carbon is diffused into the surface of the metallic material and subsequently metal carbides are formed. Carburization may result in the material becoming brittle and may cause crack formation. Furthermore, carburization can result in loss of mechanical properties, such as loss of oxidation resistance. It is also known to intentionally carburize materials to impart a high surface hardness and wear resistance. For example, steels can be treated to enrich the carbon content of the surface layer.
Metal dusting is a corrosion process which typically follows the process of carburization of a carbon source, such as carbon monoxide, typically at a temperature in the range of about 350 to about 1050° C.
In this process, metallic materials are disintegrated into a dust of graphite and metal particles in strongly carburizing atmospheres. Metal dusting may result in metal loss and/or severe pitting of the material. Without wishing to be bound by any theory, it is expected that the mechanism of carburization is as follows. First, there is uptake of carbon into the metallic phase leading to supersaturation of carbon in the alloy, which subsequently forms a metastable metal carbide (M3C, wherein M is the metal, e.g. Cr, Fe, Ni), which then migrates away from the metal surface into the graphite layer.
Material degradation caused by high temperature corrosion such as carburization and metal dusting is for example considered a problem in petrochemical technical processes, for example in heating tubes and other metal components in industrial furnaces for case hardening of steels.
Metal dusting is especially considered a problem in a reducing atmosphere, such as atmospheres containing relatively high amounts of carbon monoxide. For example, metal tubes wherein a reducing gas is processed at high temperatures may suffer from strong corrosion. Under such conditions, metal dusting causes catastrophic corrosion of the metal surfaces that are in contact with the reducing gas. Catastrophic corrosion refers to corrosion that takes place in a very short amount of time, e.g. less than 10 days.
Although mild corrosion may also be observed in cracking tubes, catastrophic corrosion by metal dusting does typically not occur to a large extent in cracking processes. The hydrocarbon gas heated in a cracking tube is not strongly reducing and does not have a carbon monoxide content higher than 0.5 vol. %. Furthermore, a the metal surfaces of a cracking furnace are typically covered by a carbon deposit, as a result of coke formation. Such a coking layer prevents the hydrocarbon gas from directly accessing the metal surface of the cracking tube, thereby limiting the corroding effect the hydrocarbon gas may have on the metal surface.
In the art, several techniques are known to prevent carburization and/or metal dusting.
WO 2010/097300 describes a surface treatment of metals to protect against metal dusting. A porous coating is used made from nanoparticles and a ceramic powder.
US 2008/0020216 describes the use of a coated metal composition comprising an oxide layer and a coating metal layer to prevent metal dusting on metallic surfaces.
An object of the invention is to provide a method for protecting metallic surfaces from high temperature corrosion in a reducing atmosphere, in particular in a carburizing atmosphere.
A further object of the invention is to provide a method for heating or cooling a reducing gas in a metal containing tube wherein the reducing gas is subjected to a high temperature at which carburisation and/or metal dusting is typically a problem in the absence of adequate protection, wherein said metal containing tube is protected against carburization and/or metal dusting.